Highways and Bridges

Highways and Bridges

The U.S. Federal Highway Administration (FHWA) has rated almost 200,000 bridges, or one of every three bridges in the U.S., as structurally deficient or functionally obsolete. Furthermore, more than one-fourth of all bridges are over 50 years old, the average design-life of a bridge.

There are 607,380 bridges in the United States (2013). Of this total, 200,000 bridges are steel, 235,000 are conventional reinforced concrete, 108,000 bridges are constructed using prestressed concrete, and the balance is made using other materials of construction. Approximately 30 percent of the bridges are structurally deficient or functionally obsolete. The annual direct cost of corrosion for highway bridges is estimated to be $13.6 billion.

Facts about bridge corrosion

Corrosion is a leading factor in the degradation of bridges. NACE International, the corrosion society, has more than 30,000 members, many of whom specialize in corrosion control in highway bridges. These members possess expertise in corrosion inspection and prevention, including protective coatings and cathodic protection.

Not all bridge collapse or bridge failures incidents are caused by corrosion, which appears to be the case in the recent Skagit River Bridge incident (5/23/13), but corrosion poses a growing threat as bridge infrastructure continues to age and spending on maintenance and repair is put off.

According to the U.S. Department of Transportation, the federal government’s annual investment is less than two thirds of what is needed to maintain roads and bridges, and this doesn’t factor in improvements. As infrastructure deteriorates the cost of maintenance and repair increases, and the longer it takes, the higher and faster those costs rise.

The Federal Highway Administration estimates it will cost $20.5 billion annually for the next 16 years to properly update existing bridges, more than 60% of what is currently being spent.

Most bridges were built for a 50 year design life, which means state highway departments will have to maintain those bridges beyond their original design lives, which will be challenging because they were built to lower design standards than those used today. The following chart shows the distribution of U.S. Bridges by Age.

Corrosion control plan for bridges

This white paper by NACE International looks at bridges and what makes them corrode. It also discusses how employing relatively low-cost corrosion control measures during initial construction can produce low-maintenance bridges with service lives of 75 to 100 years. Corrosion control protects initial bridge investment and dramatically reduces maintenance expenses in the future.

Bridge classifications

The following are definitions of terms used by the Federal Highway Administration to describe bridge conditions.

Structurally deficient

The fact that a bridge is classified under the federal definition as “structurally deficient" does not imply that it is unsafe. A structurally deficient bridge, when left open to traffic, typically requires significant maintenance and repair to remain in service and eventual rehabilitation or replacement to address deficiencies. To remain in service, structurally deficient bridges are often posted with weight limits to restrict the gross weight of vehicles using the bridges to less than the maximum weight typically allowed by statute.

Functionally Obsolete

A functionally obsolete bridge is one that was built to standards that are not used today. These bridges are not automatically rated as structurally deficient, nor are they inherently unsafe. Functionally obsolete bridges are those that do not have adequate lane widths, shoulder widths, or vertical clearances to serve current traffic demand, or those that may be occasionally flooded.

A functionally obsolete bridge is similar to an older house. A house built in 1950 might be perfectly acceptable to live in, but it does not meet all of today’s building codes. Yet, when it comes time to consider upgrading that house or making improvements, the owner must look at ways to bring the structure up to current standards.